The class of dialkyl succinates is known and widely used in low volatility organic solvents and as versatile starting materials in synthesis. A member of this class is dimethyl succinate (herein after DMS). Known methods to provide DMS include direct esterification of succinic acid. However, the high cost of succinic acid makes this method not commercially attractive.
A more cost effective source of succinic acid is derived from dibasic acid (DBA), a co-product of adipic acid manufacture. Succinic acid, however, comprises only ca. 20 wt % of DBA, the remainder being glutaric and adipic acids. As a result, the preparation of DMS from this mixed acid DBA composition, while known, provides more glutarate and adipate diesters than DMS. Often demand for glutarate and adipate diesters is less than that for the more desirable DMS. In view of increasing demand for DMS and lessening demand for production of DBA, linked to production of adipic acid, it is very desirable to find low-cost routes to DMS.
Chinese patent document CN101343211 discloses a process for making butanediol or succinic acid diester from byproducts in maleic anhydride production. Steps include esterification of these byproducts, comprising substantially solid residue, maleic anhydride residue and/or crystallized wastewater, with C1-C8 monohydric alcohols in the presence of a catalyst; followed by distillation to recover a mixture of succinate diester, maleate diester and fumarate diester as a distillate; followed by reduction with hydrogen in the presence of a copper-based catalyst at 140-210° C. and 40-80 atmospheres; and finally distilling to obtain butanediol or succinic acid diester. A disadvantage of this process is that the distillation prior to reduction with hydrogen, which is a form of purification, requires a significant capital investment cost and high energy cost because the mixture of diesters must be vaporized and condensed in order to be recovered from higher boiling compounds as an overhead purified distillate. Thus it would be desirable to avoid the need for a distillation step prior to the step of reducing with hydrogen.
Accordingly, esterification of maleic anhydride often yields a high level of dialkyl fumarate, such as dimethyl fumarate, which has a melting point of 103° C. Therefore, it is technically more difficult to handle in a production facility than pure maleate diesters.
U.S. Pat. No. 3,830,830 (Cleveland et al.; assigned to Eastman) discloses production of dialkyl esters of succinic acid from substantially pure maleic anhydride by first hydrogenating maleic monomethyl ester. Steps further include high pressure esterification at 15-5000 pounds per square inch (psig) without a catalyst. The undesirable formation of dimethyl fumarate (by isomerization of maleate) is eliminated and the reported esterification yields of 86% and 91%, which may not be high enough for an economically and commercially viable process. In addition, esterification under high pressure often requires special equipment and is costly.
There remains in the art an unmet need for a simplified method to make dialkyl succinates which is readily commercialized in an economically desirable form. The disclosures herein provide a process for converting a mixture of maleic anhydride, succinic anhydride, maleic acid, succinic acid, fumaric acid, and other components, such as co-products or residues from maleic anhydride manufacture, which is commonly not utilized but incinerated or otherwise disposed, to a series of useful succinate diesters. A major distinction from known processes includes the hydrogenation of a complex mixture from maleic anhydride production residue, containing fumarate precursors such as fumaric acid, without any prior purification. This complex mixture from maleic anhydride production residue can also be esterified to form dialkyl maleate esters and dialkyl fumarate esters and then subjected to hydrogenation to form dialkyl succinates without any prior purification.